Most laser or other optical diagnostic techniques employ focused beams to achieve high spatial resolution and/or to provide sufficiently high light intensity to obtain a prescribed signal-to-noise ratio. Additionally, if an experimental system is to be mapped by translating the optical focal spot, it is usually important for consistent results that the focal spot maintain a constant size as it is moved about the experimental area. In the prior art, experimenters have used a typical four-element zoom lens, or, as illustrated in an article by T. J. Anderson, I. W. Kay, and W. T. Peschke, presented at the 22nd JANNAF Combustion Meeting at the California Institute of Technology in October, 1985, a field lens for focusing a collimated beam to a focal spot was physically translated, thereby moving the position of the focal spot. Alternatively, an entire optical apparatus was translated, as in the jet engine application described by Eckbreth, et al. in Applied Optics in May, 1984.